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1 SUMMARY Acceptance Tests for Surface Characteristics of Steel Strands in Prestressed Concrete The predictable transfer of prestressing force from strand to concrete is essential for the reliable performance of prestressed concrete. Residual films of lubricant and other contam- inants remaining on the strand surface after manufacture have been shown to reduce the bond between the concrete and steel. A set of quality control (QC) procedures have been developed for use by strand manufacturers or their customers as part of a routine QC pro- gram to enable rapid assessment of factors that affect bond quality. The development of the QC test methods was based on an experimental program that evaluated a number of proposed methods for testing the physical and chemical properties of the strand surface. These methods were designed to be performed more quickly and eas- ily than mechanical pull-out testing. This program included limited mechanical pull-out testing and extensive surface and chemical testing. These tests, as well as transfer length tests, were conducted on a range of strand sources to establish correlations between the proposed QC tests methods and bond quality. Although concrete pull-out testing appears to correlate best with transfer length, the evaluation of correlations between the proposed QC test methods and bond quality was based on available mortar pull-out test results for the strand sources. The four test methods that showed the best correlation with bond in concrete, mortar, or both, and that are recommended for inclusion in future QC programs are as follows: · Weight Loss on Ignition (LOI) (QC-I), · Contact Angle Measurement after Lime Dip (QC-I), · Change in Corrosion Potential (QC-I), and · Organic Residue Extraction with Fourier Transform Infrared (FTIR) Spectroscopy Analysis (QC-II). The first three methods have been designated Level I QC tests (QC-I), while the fourth is a Level II QC test (QC-II). The Level I tests are quicker and less complex than Level II tests. Regression analyses on combinations of these methods also were performed to evaluate their ability to predict bond. The three combinations that showed the best correlation, based on the adjusted coefficient of determination (R2 adj.), were as follows: · Weight Loss on Ignition (LOI) & Contact Angle Measurement after Lime Dip & Change in Corrosion Potential; · Contact Angle Measurement after Lime Dip & Change in Corrosion Potential; and · Contact Angle Measurement after Lime Dip & Organic Residue Extraction (100% stearate only).

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2 The adjusted coefficients of determination for each of these combinations were higher than the coefficients of determination for the single-predictor regression models. Acceptance thresholds for two of these individual QC tests and all of the combinations were developed. These thresholds were based on: (1) prediction intervals for the regression calculated from the available data and (2) the minimum criterion for mortar pull-out stress adopted by NASPA. Thresholds for multiple-predictor regressions are not determined using the same procedure used for single-predictor regressions. Instead, the lower bound on the prediction interval must be calculated for each combination of test results. A computational tool in the form of a Microsoft ExcelTM spreadsheet had to be developed for this purpose. It is recommended that the three Level I QC tests be adopted as part of a routine QC pro- gram for strand producers. To supplement the quarterly mortar pull-out testing program currently being performed by producers supplying the domestic market, this testing should be conducted on a weekly basis for each size of strand produced. Regular QC testing would decrease the likelihood that poor bonding strand would reach the market. When lots of strand are produced that exhibit suspicious behavior, this should then prompt additional testing using the Level II organic residue extraction test and mechanical pull-out testing. The determination of thresholds for two of the individual QC tests (Contact Angle Measurement after Lime Dip and Change in Corrosion Potential of Strand) was possible based on the relationships between the QC test and the mortar pull-out test results for this sample set, and these thresholds are conservative. The available data were not sufficient to allow threshold determination for the other two individual methods with the same con- straints. The threshold determination process is governed by the prediction intervals, which are determined by the uncertainty in the regression results. Although a significant amount of work and scientific rigor has gone into the development of the thresholds, they should not be considered absolute. Additional data collected should be used to increase the statistical confidence in the bond-QC test result relationships and may allow the development of less restrictive thresholds in the future. One potential source of additional information is NASPA's quarterly pull-out testing program, which began in 2007. Conducting the recommended QC tests on the strand samples would provide valu- able information for further refining the regression relationships, even if all the samples tested demonstrated adequate or better bond properties. Another possible means for implementing these test methods is the development of process-specific regression models and thresholds. The dataset for this study included strand sources manufactured with a number of different pretreatment and lubricant processes. Limiting the data included in the regression analysis to data collected from a single manu- facturing process, such as might be done by an individual strand producer, is expected to significantly improve the correlation of the QC test methods, since the QC test results would be influenced mainly by variations in lubricant and pretreatment concentration with chemi- cal composition remaining relatively constant. This improved correlation would also permit less restrictive thresholds.